Modeling of Inline Transitions Between Different Waveguides as Impedance Inverters for the Use in Novel Filter Designs

In this paper, a novel approach for field coupled inline transitions between different waveguides is proposed. The immittance inverting characteristics of these transitions are simultaneously utilized for filter design. Based on scattering parameters, an exact relation is derived between the desired inverter constants and the even and odd mode characteristic impedances. The well-known approximation introduced by Cohn is improved. Further it will be shown that the introduced inline transitions can be effectively modeled with a contra-directional symmetrical coupler. The description of the field coupled waveguide transitions as inverters allows a "straight on design method" for general field coupled structures and accelerates the optimization process. The integration of the inline transitions into the filter design is applied for two different types of transitions, namely, the transition between a parallel-plate waveguide and a rectangular waveguide (RWG) and the transition between a microstrip line and a RWG, using a ridged waveguide section for the coupling. For both configurations a third-order bandpass filter was realized by using three coupled waveguide resonators. Two of the coupling structures were made up of inline transitions. To verify the approach and quantify losses, a filter was fabricated and its scattering parameters measured. With this new approach low-loss waveguide filters can be integrated into a planar RF circuit while minimizing space. No delicate manufacturing processes are required.